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LHC spots particle that may be new form of matter

By Maggie McKee

A long-sought fugitive has been caught at the world’s largest particle accelerator. Experiments at the Large Hadron Collider confirm that a provocative particle called Z(4430) actually exists – and it may be the strongest evidence yet for a new form of matter called a tetraquark.

Quarks are subatomic particles that are the fundamental building blocks of matter. They are known to exist either in groups of two, forming short-lived mesons, or in threes, forming the protons and neutrons that make up atomic nuclei. Researchers have suspected for decades that quarks might also bind together in quartets, forming tetraquarks, but they have not been able to do the complicated quantum calculations necessary to test the idea.

“Our computers aren’t yet big enough to solve the theory from first principles,” says Thomas Cohen at the University of Maryland in College Park. That means no one knows if the laws of physics should allow matter to clump together to form the still hypothetical tetraquark. But the latest sighting at the LHC means we are closer than ever to finding out.

“The main argument about Z(4430) was, does it exist or not?” says Tomasz Skwarnicki at Syracuse University in New York, who is a member of the team that carried out the latest work. “We came and said Z(4430) is real.”

“It’s a very good piece of work,” says BaBar spokesman Michael Roney. It seems that the particular way BaBar searched for the particle reduced its chances of a sighting. “We didn’t have enough data to have the full sensitivity,” he says.

Meson molecules

Now that Z(4430)’s existence is confirmed, the next challenge is to work out whether it really is a tetraquark.

There is at least one reason why physicists can be hopeful. The other suspected tetraquarks might simply be loosely bound pairs of mesons, says Marek Karliner, a theorist at Tel Aviv University in Israel, who was not part of the team. Z(4430) is different because its mass does not seem to allow for this.

“There aren’t any mesons at the right masses to make such a thing,” says Karliner. This points to a bona fide particle quartet, says Skwarnicki&colon; “It does make it more likely that it’s a tetraquark.”

However, one puzzling aspect remains&colon; Z(4430) decays at least 10 times as fast as previous suspects, which doesn’t fit with simple models of tetraquark behaviour, says Karliner.

Gathering more data on how this particle decays could help shed light on whether it is a tetraquark or something else. And that could help researchers get to grips with how matter behaves at the most basic scales. Could quarks bind together in even larger groups, for example? Previous hints of five-quark groupings, called pentaquarks, have mostly disappeared in recent years, but they have not been fully ruled out, says Karliner.

“What determines who can bind together and who can’t?” he asks. “It’s completely uncharted territory.”